Copyright Statement

Abstract

Auxin promotes GA biosynthesis in the aboveground parts of plants. However, it has not been demonstrated
previously that this interaction occurs in roots. To understand the interactions between auxin and GAs in these organs, we
treated wild-type pea (Pisum sativum L.) roots with the inhibitors of auxin action, p-chlorophenoxyisobutyric acid (PCIB)
and yokonolide B (YkB), and with the auxin transport inhibitor N-1-naphthylphthalamic acid (NPA). These compounds
generally downregulatedGAsynthesis genes and upregulatedGAdeactivation genes, and reduced the level of the bioactive
GA1. These effects indicate that in pea roots, auxin at normal endogenous levels stimulates GA biosynthesis. We show also
that supra-optimal levels of exogenous auxin reduce the endogenous level of bioactive GA in roots, although the effect
appears too small to account for the strong growth-inhibitory effect of high auxin levels.